Product of divisors of a number from a given list of its prime factors

Given an array arr[] representing a list of prime factors of a given number, the task is to find the product of divisors of that number. 
Note: Since the product can be very large print the answer mod 109 + 7.
Examples: 

Input: arr[] = {2, 2, 3} 
Output: 1728 
Explanation: 
Product of the given prime factors = 2 * 2 * 3 = 12. 
Divisors of 12 are {1, 2, 3, 4, 6, 12}. 
Hence, the product of divisors is 1728.

Input: arr[] = {11, 11} 
Output: 1331 

Naive Approach: 
Generate the number N from its list of prime factors then find all its divisors in O(√N) computational complexity and keep computing their product. Print the final product obtained. 
Time Complexity: O(N3/2
Auxiliary Space: O(1)

Efficient Approach: 
To solve the problem, following observations need to be taken into account: 

  1. According to Fermat’s little theorem, a(m – 1) = 1 (mod m) which can be further extended to ax = a x % (m – 1) (mod m)
  2. For a prime p raised to the power a, f(pa) = p(a * (a + 1) / 2)).
  3. Hence, f(a * b) = f(a)(d(b)) * f(b)(d(a)), where d(a), d(b) denotes the number of divisors in a and b respectively.

Follow the steps below to solve the problem: 



Below is the implementation of the above approach:

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// C++ Program to implement
// the above approach
  
#include <bits/stdc++.h> 
using namespace std;
  
int MOD = 1000000007;
  
// Function to calculate (a^b)% m
int power(int a, int b, int m)
{
    a %= m;
    int res = 1;
    while (b > 0) {
        if (b & 1)
            res = ((res % m) * (a % m))
                  % m;
  
        a = ((a % m) * (a % m)) % m;
  
        b >>= 1;
    }
  
    return res % m;
}
  
// Function to calculate and return
// the product of divisors
int productOfDivisors(int p[], int n)
{
  
    // Stores the frequencies of
    // prime divisors
    map<int, int> prime; 
  
    for (int i = 0; i < n; i++) {
        prime[p[i]]++;
    }
    int product = 1, d = 1;
  
    // Iterate over the prime
    // divisors
    for (auto itr : prime) {
  
        int val
            = power(itr.first,
                    (itr.second) * (itr.second + 1) / 2,
                    MOD);
  
        // Update the product
        product = (power(product, itr.second + 1, MOD)
                   * power(val, d, MOD))
                  % MOD;
  
        // Update the count of divisors
        d = (d * (itr.second + 1)) % (MOD - 1);
    }
  
    return product;
}
  
// Driver Code
int main()
{
  
    int arr[] = { 11, 11 };
    int n = sizeof(arr) / sizeof(arr[0]);
  
    cout <<productOfDivisors(arr,n);
  
  
 }
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// Java Program to implement
// the above approach
import java.util.*;
class GFG{
  
static int MOD = 1000000007;
  
// Function to calculate (a^b)% m
static int power(int a, int b, int m)
{
    a %= m;
    int res = 1;
    while (b > 0
    {
        if (b % 2 == 1)
            res = ((res % m) * (a % m)) % m;
  
        a = ((a % m) * (a % m)) % m;
  
        b >>= 1;
    }
    return res % m;
}
  
// Function to calculate and return
// the product of divisors
static int productOfDivisors(int p[], int n)
{
  
    // Stores the frequencies of
    // prime divisors
    HashMap<Integer,
            Integer> prime = new HashMap<Integer,
                                         Integer>(); 
  
    for (int i = 0; i < n; i++) 
    {
        if(prime.containsKey(p[i]))
            prime.put(p[i], prime.get(p[i]) + 1);
        else
            prime.put(p[i], 1);
              
    }
    int product = 1, d = 1;
  
    // Iterate over the prime
    // divisors
    for (Map.Entry<Integer,
                   Integer> itr : prime.entrySet())
    {
        int val = power(itr.getKey(),
                       (itr.getValue()) *
                       (itr.getValue() + 1) / 2, MOD);
  
        // Update the product
        product = (power(product, itr.getValue() + 1, MOD) *
                   power(val, d, MOD)) % MOD;
  
        // Update the count of divisors
        d = (d * (itr.getValue() + 1)) % (MOD - 1);
    }
    return product;
}
  
// Driver Code
public static void main(String[] args)
{
    int arr[] = { 11, 11 };
    int n = arr.length;
  
    System.out.println(productOfDivisors(arr,n));
}
}
  
// This code is contributed by sapnasingh4991
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# Python3 program to implement 
# the above approach 
from collections import defaultdict
  
MOD = 1000000007
  
# Function to calculate (a^b)% m
def power(a, b, m):
  
    a %= m
    res = 1
  
    while (b > 0):
        if (b & 1):
            res = ((res % m) * (a % m)) % m
  
        a = ((a % m) * (a % m)) % m
        b >>= 1
      
    return res % m
  
# Function to calculate and return
# the product of divisors
def productOfDivisors(p, n):
  
    # Stores the frequencies of
    # prime divisors
    prime = defaultdict(int)
  
    for i in range(n):
        prime[p[i]] += 1
      
    product, d = 1, 1
  
    # Iterate over the prime
    # divisors
    for itr in prime.keys():
        val = (power(itr, (prime[itr]) * 
                          (prime[itr] + 1) // 2, MOD))
  
        # Update the product
        product = (power(product,
                         prime[itr] + 1, MOD) *
                   power(val, d, MOD) % MOD)
  
        # Update the count of divisors
        d = (d * (prime[itr] + 1)) % (MOD - 1)
  
    return product
  
# Driver Code
if __name__ == "__main__":
  
    arr = [ 11, 11 ]
    n = len(arr)
      
    print(productOfDivisors(arr, n))
  
# This code is contributed by chitranayal
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// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
  
class GFG{
  
static int MOD = 1000000007;                     
  
// Function to calculate (a^b)% m
static int power(int a, int b, int m)
{
    a %= m;
    int res = 1;
    while (b > 0) 
    {
        if (b % 2 == 1)
            res = ((res % m) * (a % m)) % m;
  
        a = ((a % m) * (a % m)) % m;
        b >>= 1;
    }
    return res % m;
}
  
// Function to calculate and return
// the product of divisors
static int productOfDivisors(int []p, int n)
{
      
    // Stores the frequencies of
    // prime divisors
    Dictionary<int,
               int> prime = new Dictionary<int,
                                           int>(); 
  
    for(int i = 0; i < n; i++) 
    {
        if(prime.ContainsKey(p[i]))
            prime[p[i]] = prime[p[i]] + 1;
        else
            prime.Add(p[i], 1);
    }
    int product = 1, d = 1;
  
    // Iterate over the prime
    // divisors
    foreach(KeyValuePair<int,
                         int> itr in prime)
    {
        int val = power(itr.Key,
                       (itr.Value) *
                       (itr.Value + 1) / 2, MOD);
  
        // Update the product
        product = (power(product, itr.Value + 1, MOD) *
                   power(val, d, MOD)) % MOD;
  
        // Update the count of divisors
        d = (d * (itr.Value + 1)) % (MOD - 1);
    }
    return product;
}
  
// Driver Code
public static void Main(String[] args)
{
    int []arr = { 11, 11 };
    int n = arr.Length;
  
    Console.WriteLine(productOfDivisors(arr,n));
}
}
  
// This code is contributed by PrinciRaj1992
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Output: 
1331

Time Complexity: O(N) 
Auxiliary Space: O(N)
 

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